Researchers showed a new microscopy technique allows researchers to track microstructural changes in real time, even when a material is exposed to extreme heat and stress. A stainless steel alloy called alloy 709 has the potential for elevated temperature applications such as nuclear reactor structures.

A new microscopy technique allows researchers to track microstructural changes in real time, even when a material is exposed to extreme heat and stress. Recently, researchers show that a stainless steel alloy called alloy 709 has the potential for elevated temperature applications such as nuclear reactor structures.

Alloy 709 is exceptionally strong and resistant to damage when exposed to high temperatures for long periods of time. This makes it a promising material for use in next-generation nuclear power plants.

However, alloy 709 is so new that its performance under high heat and load is yet to be fully understood. And the Department of Energy (DOE) needed to understand its thermomechanical and structural characteristics better to determine its viability for use in nuclear reactors.

Scanning Electron Microscope

To address DOE's questions, Rabiei came up with a novel solution. Working with three companies Hitachi, Oxford Instruments and Kammrath & Weiss GmbH, Rabiei developed a new technique that allows her lab to perform scanning electron microscopy (SEM) in real time while applying extremely high heat and high loads to a material.

This means we can see the crack growth, damage nucleation and microstructural changes in the material during thermomechanical testing, which is relevant to any host material not only alloy 709. It can help us understand where and why materials fail under a wide variety of conditions: from room temperature up to 1,000 degrees Celsius (C), and with stresses ranging from zero to two gigapascal.

To place that in context, 1,000 C is 1,832 degrees Fahrenheit. And two gigapascal is equivalent to 290,075 pounds per square inch. Rabiei's team collaborated with the University of Birmingham in the United Kingdom to assess the mechanical and microstructural properties of alloy 709 when exposed to high heat and load.

Samples

The researchers exposed one-millimeter-thick samples of alloy 709 to temperatures as high as 950 C until the material "failed," meaning the material broke. Alloy 709 outperformed 316 stainless steel, which is what's currently used in nuclear reactors. The study shows that the alloy 709's strength was higher than that of 316 stainless steel at all temperatures, meaning it could bear more stress before failing. 

Microscopy Technique

And our microscopy technique allowed us to monitor void nucleation and crack growth along with all changes in the microstructure of the material throughout the entire process. This is a promising finding, but we still have more work to do. Our next step is to assess how alloy 709 will perform at high temperatures when exposed to cyclical loading, or repeated stress.